1. Field of the Invention
The present invention relates to a display element and a method of manufacturing the same. More particularly, the present invention relates to a thin-film transistor (TFT) comprising a copper electrode and a method of manufacturing the same; and especially relates to forming a barrier layer only onto the semiconductor layer of the transistor by using an electroless plating process with deposition selectivity.
2. Descriptions of the Related Art
As liquid crystal display (LCD) panels gradually increase in size, the resolution thereof needs to increase accordingly. As a result, aluminum has been gradually being replaced by copper as the conductive material in the manufacturing process of the LCD panels. This is because copper has many advantages over aluminum, such as a lower resistivity, a lower thermal expansion coefficient, a higher melting point, a higher thermal conductivity and a greater resistance to electro-migration. Therefore, copper conductors may lead to a display panel with a higher circuit density and a more superior imaging quality, and may also reduce the manufacturing cost thereof. As a result, the manufacturing process adopting copper conductors has become common and popular for large-sized high-resolution LCDs.
However, a number of problems still exist in the existing LCD manufacturing processes when copper is adopted as the conductive material. Some examples include: the inability to form a self-protective oxide layer, poor adhesion with the dielectric layer, a high diffusion coefficient in the semiconductor layer and the dielectric layer, and reaction with silicon at a low temperature which forms a silicide. All these problems may lead to the degradation of electrical performance of such conductors in the LCDs, thus causing an adverse impact on the quality of LCDs.
In order to alleviate such problems, copper is generally used in combination with a barrier layer in the art at present. For example, when copper is used as the source/drain electrode of a TFT, a barrier layer is typically disposed between the copper and a semiconductor layer to prevent an undesired diffusion effect due to the direct contact therebetween. Such a barrier layer is typically made of nickel (Ni), tantalum (Ta), titanium (Ti), molybdenum (Mo), chromium (Cr), tungsten (W), or an alloy thereof.
FIG. 1 depicts a schematic cross-sectional view of a conventional display element adopting such a combination. In this display element, a transistor region 111 and a capacitance region 113 are defined on a substrate 11. The transistor region 111 has a transistor structure formed therein, which comprises in sequence (from bottom to top): a gate 131, a dielectric layer 15, a patterned semiconductor layer 17, a patterned barrier layer 19, a source/drain electrode 211, a passivation layer 23, and a patterned pixel electrode 25. The patterned semiconductor layer 17 further comprises a channel layer 171 and a source/drain layer 173. The capacitance region 113 comprises in sequence from bottom to top a first conductor 133, a dielectric layer 15, a patterned barrier layer 19, a second conductor 213, a passivation layer 23, and a patterned pixel electrode 25. The gate 131 and the first conductor 133 are respectively a portion of a first patterned conducting layer 13, while the source/drain electrode 211 and the second conductor 213 are respectively a portion of a second patterned conducting layer 21. The second patterned conducting layer 21 is made of copper or its alloys, although other conductive metal materials such as aluminum may also be optionally used.
In the structure depicted in FIG. 1, as described above, a barrier layer 19 is formed between the source/drain electrode 211 made of copper and the semiconductor layer 17 for separation therebetween, which is accomplished by a usual deposition and followed with lithographic and etching processes. Although this may overcome the diffusion problem described above, the barrier layer 19, which typically has a much higher resistivity than the conducting layers 13, 21, will increase the overall resistance significantly. Actually, the barrier layer 19 is also formed in some unnecessary regions of the display element such as the region circled by the dashed line. However, due to the limitation of lithographic and etching processes or out of convenience, the presence of the barrier layer 19 in these unnecessary regions is typically unavoidable. Therefore, elimination of such an unnecessary dual-layer structure in these regions can prevent an unnecessary increase in the resistance, thereby improving the performance of the resulting transistor.
Additionally, when the lithographic and etching processes, especially a wet etching process is performed, differential electrochemical reactions, i.e. different etching rates, to copper and the barrier layer tend to occur, and resulting in an undercutting to the barrier layer 19 at the periphery of the second patterned conductor 213. Such an undercut may cause degradation of the electrical performance of the thin-film transistor (TFT) and render it difficult to control the width of the conductor.
Although the formation of a barrier layer under the copper electrode in the existing TFT manufacturing process may obviate the diffusion effect, it will unnecessarily cause an increased resistance and undercut the barrier layer. In view of this, it is highly desirable to provide a method for manufacturing a display element that can eliminate all of the problems mentioned above.